Wednesday, September 26, 2012

Project Clean Up Part 1 Chapter 6

Chapter 6: "After Action Report and Rebuild Plans"

This is a continuation, the First part is Here,  a complete list is available under "Ham Radio Master links" on the sidebar.

So Here's what I have been able to discover:
I'm still not 100% certain why the unit failed the first time. Initially I suspected the power supply, but I'm reinvestigating, I discovered another loose solder connection on one of the tube sockets. I'm not sure that's what did it, but I'm always suspicious of any changes I made. I changed tubes, the supply blew up. Thems the facts. Whether changing the tube caused a supply malfunction remains to be seen.

You can notice from the pic that I changed out the crystal holder part of the circuit. I did this to keep the metal case of the crystal WELL away from the supply voltage terminal, and because it looks better put together up and off of things as opposed to flopping around all dangly like. The bad news is my thumb is sore today because I think I poked it with a wire. If things don't improve soon, may have to go see the Dr about lockjaw! Heh!
how you 'spain that one to the XYL?
The second time the supply failed, I am 100% certain was because I used a severely underrated rectifier diode. I used a 1N4001 blindly following a misprint in the schematic I was using, instead of a 1N4004. So I'm beefing up to a 1N4005 just to make sure, and that's only IF I can't find any 1N4007's laying around the junque box. Next I will isolate the 150 B+ part of the circuit from the line, and instead of using one diode, I will go ahead and use four, building a full wave bridge rectifier. This is because I got diodes, and math is on my side...

From an awesome mess
Comes a beautiful circuit
Arise O Phoenix!

so you take a half wave rectifier like W3IRZ(sk) first designed into this rig, and you take the caps he had for smoothing, do some funny math, and using the max transformer rating of 1 amp. I'm not expecting to actually draw more than about .05 amp here, and yes, I know about derating the transformer current rating in a bridge rectifier, the actual rating is 1.2 Amp. Radio Shack sells a 3 Amp transformer too, it is part number 273-1511 you can ship it to my address from or hit the donate button in the top right hand corner of this webpage :)
you do the math found at MIT: and discover that there is 24.1 Volts of ripple voltage when the load draws 1 Amp.
Now this is worst case scenario mind you, but still. at 50mA which is a lot closer to the actual current draw, it will be a lot less, like  1.205 volts ripple, which is less than 1% of peak voltage (150VDC). An acceptable number, but since the 470 microfarad caps needed to produce this performance are now BBQ in the trashcan, it's time for plan B.
I have some caps suitable for filtering, but not the exact values as before. I needed a way to lower the required amount of smoothing needed to get a usable DC voltage. I decided to build a bridge rectifier, and use a couple of caps left over from the earlier efforts. Using two 220 microfarad capacitors, I actually drop my ripple voltage a little bit, to like .95 volts at 50mA. I can handle that!
Now you can buy a bridge rectifier at the local shack, but this is an exercise in understanding, and I don't have any 1 Amp, 120Vrms AC bridge rectifiers in the Junque box. I do have 4 1N4005 diodes though, and  knowledge, and a community of assorted skalawags, errrr, electrically inclined persons who can help me out.

This is the bridge, as I have constructed it. The alligator grippers are holding it at the position where I hook in the AC, one side to top, one side to bottom.
The bridge will be hooked the  (originally primary) secondary of a 12.6VAC to 120VAC transformer.

Here's a closeup, showing the AC wires hooked into the circuit. I've intentionally left the 'ears' on the soldered circuit leads so I can do some testing later. You can see that the bands indicating the cathode of the diode are running from left to right. As it is layed out, the rectified positive voltage will be on the right, and the negative side is the left.

This is an overhead shot of the circuit so far, laid out on a potential housing mount. The pic is upside down from the way that I took it, but whatever.
you see that I have two identical transformers, they are Radio Shack "120-12.6VAC 1.2 Amp" available at most stores. I have rolled up and taped off the center tap of the transformer, because I don't plan on using it. The two outer leads have been tied together. I will take my 12 VAC filament voltage from this point. Hot and neutral wires will be hooked up to the primary transformer on the left, and the diode bridge is hooked up to the secondary transformer on the right. If all goes well, and I haven't done anything stupid, we will have pulsed DC ready for smoothing at the secondary end.
Here's a video showing how everything is currently laid out:
I won't be hooking AC up to it until I've got the unit planned out and tightened up a lil bit, namely I will be solving space issues, hooking up a switch and fuse set up, and hooking it up the way I will have it inside the box.
So what do you guys think?
Go or No Go?

Click here to continue to Chapter 7: "Persnickety Power Problem Pwned and Pacified"

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